Electrical musical instrument



y 1950 L. E. A. BOURN 2,507,884

ELECTRICAL MUSICAL INSTRUMENT Filed Aug. 7, 1947 7 \i U/ \l s s 1 L Q L M v g E u S Q i Q n t Q: 2 2 2 0) LG) Q g g Q Q Q Q m Y 2 s g '3 x a 5 u Inventor Attorney This invention relates strum'ents of the kind comprising amultiplicity Patented May 1 6, 1950 UNITED STATES PATENT OFFICE Application August "I, 1947, Serial No. 767,202

.In Great Britain January 15, 1945 Section 1, Public'Law 690, August 8, 1946 Patent expires January 15, 1965 2 Claims. 1

to electrical musical ,in-

of frequency generators which are brought into operation selectively, say by the action of stops and keys. More particularly the invention relate's to those arrangements in which the control of the 'fre'quencygenerators is efiected by the application of direct current potential. and/or current either direct to the generators themselves (as in our prior British specifications Nos. 403,444 and 433,050) or to control devices'ass'ocia'ted with the'g' enerators. .Such control devices can take the form either of valves (as incur cpending British specification No. 23,207/44) or of some other form of direct current controlled a1 7 ternator.

In such arrangements, resistance-condenser networks or low-pass filters have been proposed for preventing an audible key click from being sounded upon depressionof a key-owing to the production of a sudden transcient. Such low-pass filters have also been proposed which also have a second function, viz. that of modifying and controlling the rate of attack and die-away. The use of rectifiers has also been proposed for the same purpose.

It is not easy to make the attack quick enough to'be imitative of a read, for instance without a low pitched thud being audible owing tfo'the fact that the build-up forms the first halt-cycle of a low frequency note which, particularly it loud-speakers are used which are not critically damped, gives rise to a damped train of oscillations. If the time-constant of the build-up is decreased, the pitch of the thud rises until the thud becomes a click." If the thud is to be completely avoided the time-constant of the build-up must be such that the half-cycle formed by said build-up is of a frequency which is below audibllity. If this is the case the build-up is too slow for many musical purposes.

The object of the present invention is to provide means by which a simple single or multistage low-pass filter may be used without the above disadvantage.

The invention consists broadly of an electrical musical instrument of the kind comprising a multiplicity of frequency generators which are brought into operation selectively, say by the action of stops and keys, by the application of direct current potential and/or current either direct to the generators themselves, or to control devices associated with said generators, and in which low-pass filters are associated with the control circuits by which said direct current potential or current is applied, wherein the generators are divided into a plurality of groups of difierent frequency ranges, and wherein the group, or each group, at least of higher frequency range, has a high pass or band pass filter associated with it which permits the wanted frequencies to pass but filters out the sound which would otherwise result from the rate of attack permitted by the low-pass filters.

In the accompanying drawing is shown a block schematic diagram of an embodiment of the invention as applied to an instrument of the smaller order, in which say frequencies are generated by electrostatic generators as described in British specification No. 23,207/44, and in which thermionic valves are used to control the strength oi the notes, through the control of bias voltage by the keys and stops, the anodes of said valves being fed through a common anode load resistance, and the collective outputs from said anodes being fed to a common amplifier and loud-speaker. As shown in the drawing, the bias voltage source H) is subject to the control of control circuits II in response to the keys and stops of the instrument. When the invention is applied to such an instrumentall valves associated'with'generators I2a which generate frequencies within the compass of say the bottom three octaves, i. e. frequencies ifrom 32.625 to 261 cycles per sec.,

may be grouped in one channel and fed into a common amplifier 13c and loud-speaker Main the ordinary manner, but the filters 15a associated with the keying circuits forsaid valves are arranged. so that the voltage build-up is too slow to produce a thud at the commencement of the sounding of the note. This, although it gives an attack which is sluggish from a musical point of view, detracts but little from the general performance of the instrument. Large amplitude surges in the loud-speakers are also avoided by thus providing for a slow attack. The collective outputs from the valves associated with generators I2b covering the next three octaves of irequencies are similarly grouped together and fed into a second amplifier I3a and loud-speaker Ila. A high-pass or band pass filter Ifib is incorporated in the amplifying chain preferably between the control valves and the amplifier proper, said filter being of such characteristics that only the wanted frequencies will be passed, and particularly irequencies lower than the lowest note of the group are suppressed or attenuated. The filters lib associated with the keying circuit of this group are made with a much shorter build up period, which, when operated, would produce a pronounced low frequency thud were it not for the presence of the said high or band pass filter. The remainder of the valves may be treated inthesame manner as the last or mid-frequency group with generators I20, amplifier I30 and loud-speaker I40 but with the high-pass or band-pass filter I60 suitablydesigned to suit the frequency range; and the'filters [50 associated with the keying circuits made with a still shorter build-up period, though not short enough to pass the high or band pass filter.

In instruments where complex tones are generated and where even in the lowest frequency group the frequency range may extend upwards to the limit of audibility, the filter employed in the amplifying chain, must be of the high-pass variety only, as the use of band-pass filters would attenuate the higher frequencies associated with the tone.

In cases where control valves are not used as for instance in British specification No. 403,444, and in any other cases where a surge is produced the depression or operation of a key, the outputs from the various generating units may be divided and controlled in the manner described above.

"As has been mentioned, two such groups are usually sufiicient to meet most requirements, and the exact division point as a matter of opinion, but generally speaking the lowest three octaves may be grouped-to form the bass channel and the remainder groupedto form the treble channel.

By means ofthis sub-division into groups, it is possible to suppress the thud in the parts of the compass where promptitude of attack is necessary, and in the bass the thud is avoided 0wing to the fact that lessprompt attack does not impair the musical effect.

It is to be understood that it may in some cases be of advantage also to include a high or band pass filter in the output of the lower frequency group as well as in the output of each of the higher frequency groups.

We claim: a 4

An electrical musical instrument comprising a plurality of frequency generators whose frequencies lie within -a given range, control circuits associated with said generators, control means whereby direct current potential is adapt-v ed'to be selectively applied to said control cir-- cuits, means whereby said generators are rendered operative to generate said frequencies by the application of such direct current potential tosaid controlcircuits, sound-producing appa- 4 ratus, output circuits associated wtih said gerierators for conveying the generated frequencies to said sound-producing apparatus for causing the latter to produce the corresponding sound tones, low-pass filter means connected to said control circuits for blocking frequencies within the aforesaid range while permitting a given rate of attack of said direct current potential, and filter means connected to said output circuits for giving passage to frequencies within the aforesaid range while blocking frequencies generated by said rate of attac of said direct current potential.

2. An electrical musical instrument comprising a plurality of frequency generators, control circuits associated with said generators, control means whereby direct current potential is adapted to be selectively applied to said control circuits, means whereby said generators are rendered operative to generate said frequencies by the application of such direct current potential to said control circuits, sound-producing apparatus, output circuits associated with said generators for conveying the generated frequencies to said sound-producing apparatus for causing the latter to produce the corresponding sound tones, said generators being divided into a plu railty of groups of different frequency ranges, including a relatively low-frequency group and a relatively high-frequency group, low-pass filter means connected to the control circuits of the low-frequency group for blocking frequencies above audibility, low-pass filter means connected to the control circuits of the high-frequency group for blocking frequencies within the range of said high-frequency group while permitting a given rate of attack of direct-current potential in the control circuits of said high-frequency group, and filter means connected in the output circuits of said high-frequency group for giving passage to frequencies within the range of said high-frequency group while blocking frequencies generatedby said rate of attack of said direct current potential.

LESLIE EDWIN ALEXANDER BOURN.

REFERENCES CITED The following references are of record in the file of this patent:

Hanert Sept. 2, 1941 

